Students have little opportunity to observe or experiment with blood filtration in the nephron. Thus, we have developed a modeling activity on the blood filtration in the nephron for middle school students. The students present their mental models of the principles of blood filtration in small groups. They then participate in a hands-on activity to conduct the blood filtration process using a syringe filter, then revising their initial models based on the activity and data analysis. Through this modeling activity, the students can build their knowledge about the excretion principle.

Introduction

The process of excretion is an invisible, abstract concept, which makes it difficult for students to learn about (Clement, 2003). Students rarely have an opportunity to observe the blood filtration process in their classrooms. As a result, they tend to simply conceptually memorize the process of blood filtration in the kidneys.

Modeling activities, in which students make mental models and verify or revise their models through experiments, help students to understand the principle involved (Gilbert et al. 2000). Developing models and predicting unobservable phenomena are also emphasized in the Next Generation Science Standards (NGSS Lead States, 2013).

We have developed a lab activity that allows students to experience the pressure of filtration and the filtration of materials through semi-permeable membranes. Based on this lab activity, the students are asked to construct and modify a model that describes how nitrogenous wastes in the glomerular blood are filtered into the Bowman's capsule. Our activity has the following advantages. First, it provides an opportunity for students to explicitly express their vague ideas about the principles of nitrogenous waste filtration. The students’ alternative concepts can be revealed and modified into scientific concepts at the end of the activity (Buckley & Boulter, 2000). Second, it contains a simple hands-on syringe filter activity. The students go beyond simply performing the activity and use experimental experience to evaluate their initial models. Finally, the students can develop their models closer to the scientific model in small group discussions. Critical discussion among students is also emphasized in the NGSS (NGSS Lead States, 2013).

This hands-on activity provides students with a figurative experience of how the blood is filtered from the glomeruli to the Bowman's capsule. It is necessary for the students to learn the structure of the excretory organ prior to this activity so that they can map each element of this activity properly with the target concept. After this lesson, the students have the opportunity to construct their model to explain the process of urine production through reabsorption, as well as the secretion process between tubules and capillaries.

Preparation

Materials (per group of 4 students)

  • 10 mL potato starch solution (1%)

  • 10 mL dextrose solution (1%)

  • 1 syringe filter (nylon, pore size 0.2 μm)

  • 1 syringe (10 mL)

  • 10 mL dropping bottle of iodine–potassium iodide solution

  • 10 mL dropping bottle of Benedict's reagent

  • 1 beaker (50 mL)

  • 3 test tubes (18 mL)

  • 4 dropping pipets

  • 1 glass rod

  • 1 tube rack

  • 1 test tube holder

  • 1 alcohol lamp

Activity Procedure

The activity consists of three stages, as depicted in Figure 1. A detailed description of each stage and important points are described below.

Figure 1.
Overview of the three stages of the activity.
Figure 1.
Overview of the three stages of the activity.

Stage A: Model Construction (15 minutes)

  • 1.

    After organizing small groups of four students, the teacher can introduce a discussion as follows:

    • A large amount of urea in the renal arteries is eliminated from the glomeruli and excreted in urine, leaving a small amount of urea in the renal vein.

    • How is the urea eliminated from the glomeruli?

  • 2.

    The teacher shows the students Table 1 and asks them to choose one of the three models that describe how nitrogenous wastes are separated from the glomeruli. After selecting a model, the students in each group have a short discussion regarding why they chose that model.

Table 1.
Three tentative models.
A. Similar to the way in which only iron powders are separated by the magnet, only wastes in the blood are extracted by the pulling force of the kidney. B. Since dense substances sink, nitrogenous wastes are heavy and sink in the blood. C. Similar to the way in which muddy water is filtered through the paper, small substances, nitrogenous wastes, are filtered through the membrane. 
   
A. Similar to the way in which only iron powders are separated by the magnet, only wastes in the blood are extracted by the pulling force of the kidney. B. Since dense substances sink, nitrogenous wastes are heavy and sink in the blood. C. Similar to the way in which muddy water is filtered through the paper, small substances, nitrogenous wastes, are filtered through the membrane. 
   

Importance of This Stage

The students express the ideas they already have about blood filtration. The teacher should encourage them to talk with their peers, rather than assessing whether the model they have chosen is right or wrong. At this stage, the students need to express their ideas explicitly so that they can modify them into scientific models in Stage C.

Stage B: Syringe Filter Activity (25 minutes)

  • 3.

    The teacher can introduce the syringe filter activity as follows:

    • We will conduct an experiment demonstrating the process of blood filtration in the glomeruli. You need to consider how this experiment relates to blood filtration in the kidneys.

  • 4.

    The students in each group perform the following experiment. First, they add 10 mL of the potato starch solution (1%) and 10 mL of the dextrose solution (1%) to a 50 mL beaker and mix. They then add 5 drops of the iodine–potassium iodide solution to the beaker. Next, they remove the needle part of the syringe and draw in 7mL of the mixed solutions. Finally, the students install the syringe filter at the inlet of the syringe.

  • 5.

    The students should position the syringe hole so it faces the table. The teacher asks the students following question:

    • Does the solution in the syringe come out? The students observe that the solution is not filtered by gravity alone.

  • 6.

    The students place one test tube under the syringe and press the syringe. The teacher can introduce a discussion as follows:

    • We observed that the solution needs a pressing force to be filtered. Which force in the kidney makes it possible to filter the blood? Students can realize that the high blood pressure of glomeruli enables blood filtration.

  • 7.

    The students observe that the color of the solution in the test tube differs from the solution in the syringe. The teacher asks them to think about the following:

    • What is the difference between the components of the two solutions?

    • What tests can we use to identify the differences in the components?

  • 8.

    The students design and conduct an experiment to determine whether any of the substances in the syringe could come out (10 minutes). The following data may be used:

    • Starch + Iodine-potassium iodide solution → Blue solution

    • Glucose + Benedict's reagent + Heat → Yellowish-red solution

  • 9.

    The students connect each part of the syringe filter activity to the part of the kidney it represents by referring to Figures 2 and 3 (5 minutes).

    • Example: Pressure: Blood pressure in the glomerulus

      Potato starch: Red blood cells, Protein

      Glucose: Water, Glucose, Amino acids, Urea

      Syringe filter: Cell membrane of the glomerulus

      Test tube: Bowman's capsule

Figure 2.
Elements of the syringe filter activity.
Figure 2.
Elements of the syringe filter activity.
Figure 3.
Part of the nephron and substances in blood.
Figure 3.
Part of the nephron and substances in blood.

Importance of This Stage

The teacher should encourage the students to focus on linking the syringe activity to blood filtration in the kidneys. The students observe that the blood needs to be pressed in order to be filtered, and small substances are filtered. This stage can be used as an experimental foundation when the students revise their initial models in Stage C.

Stage C: Model Revision (20 minutes)

  • 10.

    The teacher distributes Table 2 to the students, explaining what is shown so that the students can evaluate their initial models. Table 2 shows whether the substances in the blood of the glomeruli were removed into the Bowman's capsule as well as the characteristics of each substance (5 minutes).

  • 11.

    In small groups, the students evaluate their initial models using the experimental results and data in Table 2. Group opinions are presented during a whole class discussion (10 minutes). The students may evaluate three models as follows:

    • Model A: In Table 2, not only wastes but also water, glucose, and amino acids are removed from the blood, so it cannot be said that only wastes are filtered out.

    • Model B: In Table 2, substances with a low molecular weight are also filtered, so the substances are not separated by density differences.

    • Model C: In the syringe filter activity, the small-sized glucose is filtered, but the large-sized starch is not filtered. In Table 2, the substances that are filtered are small molecules. In Model C, the materials are filtered by gravity, but blood filtration requires pressure.

  • 12.

    During the last 5 minutes, students form a model of the blood filtration process from the glomeruli to the Bowman's capsule. The students can take note of following:

    • There are small holes in the glomerulus, and filtration occurs as the blood passes through them.

    • Similar to the way in which the starch is not filtered in the syringe filter activity, only small-sized substances can be filtered through the holes.

    • Small substances pass through the syringe filter when pressure is applied in the syringe. The filtration of nitrogenous wastes also requires pressure (blood pressure).

Table 2.
Removal of substances in the blood of the glomeruli into the Bowman's capsule, as well as their characteristics.
Substances in the blood of glomeruliRemoval into the Bowman's capsuleRelative size of molecules/cellsEnergy sourceToxic
Water Yes 18 No No 
Glucose Yes 180 Yes No 
Amino acids Yes 142 Yes No 
Urea Yes 60 No Yes 
Red blood cells No 1.9 × 1010 No No 
Protein No 1 × 106 Yes No 
Substances in the blood of glomeruliRemoval into the Bowman's capsuleRelative size of molecules/cellsEnergy sourceToxic
Water Yes 18 No No 
Glucose Yes 180 Yes No 
Amino acids Yes 142 Yes No 
Urea Yes 60 No Yes 
Red blood cells No 1.9 × 1010 No No 
Protein No 1 × 106 Yes No 

Note: The relative size of molecules/cells is determined by their molecular weight when the atomic weight of hydrogen is 1. The energy source column indicates whether the substance is an available energy source in the cell.

Importance of This Stage

The students use the experimental results of Stage B and the data in Table 2 to find the limitations of their initial models. They then have the opportunity to critically analyze the models while discussing them with their peers. Finally, they can construct a scientific model by modifying their initial model.

Conclusion

This activity provides students with an opportunity to construct and modify the model of blood filtration in the nephron highlighted in the NGSS. This activity is simple and does not require sophisticated equipment. The students can organize their knowledge by participating in the modeling activity rather than simply memorizing the principles. This activity was designed for middle school students but can be used in other grades depending on the teachers’ implementation. For instance, adding an experiment using a syringe filter with a larger pore size (0.45 μm) could allow students to discuss renal disease with proteinuria in the kidneys.

References

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